1. Field of Invention
This invention relates microporous, polyhalogenated, polymeric films. More particularly, this invention relates to polyhalogenated, polymeric films, as described hereafter, which are prepared utilizing micro powders having particle sizes of less than about two microns.
2. Prior Art
Microporous films are used in a wide range of applications, generally to provide barriers which are selective to the passage of various materials such ions or molecules. For example, microporous films may be used as battery separators, ion-exchange membranes and electrolysis membranes, as well as in breathable fabrics and medical and packaging applications. Many prior art films have been formed from polyolefins which can be rendered microporous by the extraction of a soluble component or by other processes. Among the polymers which can conveniently be rendered microporous by these processes are included polyethylene and polypropylene. Films of these polymers, while chemically inert towards many acids, alkalis and reactive metals, are, none-the-less, incompatible with a number of solvents which are commonly used. Moreover, the maximum operating temperature for polyolefins of this type is approximately 120.degree. C. As a result, these films cannot be employed where high temperature performance is necessary.
The preparation and use of porous, polyfluorinated, microporous films is known. For example, U.S. Pat. No. 3,407,249 discloses a porous polytetrafluoroethylene (Teflon.RTM.) film useful as a battery separator. The patent describes the use of Teflon particles having diameters in the range of about 0.1 to 0.5 microns. According to the patent, these particles are mixed with a polymer such as polymethylmethacrylate, hot-milled and extruded. The polymethylmethacrylate polymer is then dissolved from the extruded film to produce a microporous product.
The microporous films prepared in the '249 patent are significantly different from the films of the instant invention in that they are "extensively fibrillated" , i.e., the pores of the films of the '249 patent are in reality slits or "cracks" in the film. Thus, the slit pores of the films of the '249 patent do not lend themselves to the uniform separation of ionic materials such as is necessary with battery separators. Moreover, the films prepared in the '249 patent are primarily based upon polytetrafluoroethylene films and do not possess the superior properties of the films prepared according to the instant invention.
U.S. Pat. No. 2,806,256 discloses the preparation of battery separators based on polytetrafluoroethylene and polytrifluoromonochloroethylene polymers. There is no disclosure in this patent to the use of the specific polymers disclosed as useful herein.
Other prior art, microporous, polyfluorinated polymers, like the materials described in the '249 patent, have suffered from a lack of uniform pore size distribution. While the films of the instant invention offer a pore size distribution of about 0.1 to 1 microns, the pore size distribution shown in prior art films is not nearly as narrow nor are the pores as small; the average pore diameter range of many prior films ranging from about 1 to 5 microns. Another important characteristic present in the films of the instant invention, but found lacking in prior art films, is transverse direction, tensile strength. Prior art membranes have generally been extremely weak in the transverse direction due either to their highly fibrillated or highly drawn nature.
U S. Pat. No. 3,929,950 discloses the formation of porous synthetic films based upon ethylene/tetrafluoroethylene copolymers (ETFE). In the '950 patent another polymer such as polypropylene is blended into the polymer melt and then dissolved out again, after film formation, to produce a microporous film. However, the '950 patent does not disclose the particle size of the ETFE copolymer which is utilized. Also, because of the extensive stretching required, the films described in this patent are anisotropic, highly oriented and accordingly have relatively wide, non-uniform pores. The films of the instant invention, on the other hand, exhibit a very narrow pore size distribution and, at the same time, the films are not anisotropic in nature.
Other patents which disclose polyfluorinated microporous films include U.S. Pat. Nos. 4,702,836, 3,661,645, 3,679,614, 4,623,670 and 4,342,636.
Other prior art process have used ionic materials, particularly lithium salts in rendering polyhalogenated films microporous. However, the presence of even trace amounts of these ionic salts in certain end uses for microporous films such as battery separators may be deleterious. Moreover, many lithium salts are hygroscopic and therefore tend to undesirably attract and hold water within films prepared utilizing these salts. As a result, dielectic breakdown of battery separators made from microporous films based upon lithium salts may occur. Films of the instant invention, on the other hand, are not prepared based upon ionic salts and accordingly do not suffer from the disadvantages demonstrated by many prior art polyhalogenated microporous films.
Accordingly, it is an object of this invention to prepare microporous films having relativity uniform pore sizes.
It is another object of this invention to prepare polyhalogenated microporous films having relatively small pore diameters.
It is another object of the instant invention to prepare essentially ion free films which do not contain residues of ionic salts.
It is also an object of this invention to prepare microporous polyhalogenated films having improved transverse direction strength when compared to prior art, polyhalogenated, microporous films.
It is also an object of this invention to prepare microporous films which are not highly oriented and which are based upon polyhalogenated copolymers.
It is yet another object of this invention to prepare films which are useful in high temperature applications and are substantially inert to chemically aggressive substances such as alkali and alkaline earth metals.
These and other objectives are obtained by preparing the compositions of the instant invention utilizing the process described below.